568 ON INCREASE IN COMPLEXITY [pt. iii 



ment here made may be differently expressed by saying that pre- 

 formationism is only true in the sense that the constitution of the 

 fused nucleus must have reference to, though it dots not actually 

 resemble, the adult form. Protoplasm, and yolk, on the other hand, 

 as Lillie on Chaetopterus, Morgan & Spooner on Arbacia, and Morgan 

 on Cumingia have shown, are homogeneous as far as this goes, since 

 normal development can take place after centrifugation and dis- 

 placement of visible constituents. (See p. 346.) 



It is known that eggs vary a great deal as to their dependence on 

 the arrangement of the initial material in the egg-cell. The extent 

 to which they do so is more or less ascertainable by the extent to 

 which they exhibit totipotence and the rapidity with which they lose 

 it. The phenomenon of totipotence was originally discovered by 

 Driesch in 1900, who found that, if the blastomeres of the sea- 

 urchin's egg were separated at the 2-cell stage, each nevertheless 

 gave rise to a perfect larva of half the normal size. If the blastomere 

 was taken from the 4-cell stage it would produce a larva of a quarter 

 the normal size. Blastomeres from the 8-, 16- and 32-cell stages did 

 not gastrulate well, and not at all if they were derived from the 

 animal pole. No blastomere from the 64-cell stage will produce a 

 gastrula. However, although one blastomere from the i6-cell stage 

 animal pole will not gastrulate, four together will, and will go on 

 to produce a perfect larva, so that the inability cannot be due to lack 

 of specific enteron-forming substance, but simply to too small size. 

 The gradual loss of totipotence, therefore, is an index of the degree 

 to which the egg depends on the localisation of organ-forming 

 materials in the original egg-cell. In some animals the blastomeres 

 are able to regulate their development, if isolated, and produce a 

 perfect larva, in other animals they are not: the former are termed 

 "regulation eggs" and the latter "mosaic eggs". Examples of the 

 former group, besides the echinoderm eggs just mentioned, are the 

 coelenterates, which, according to Zoja, can be totipotent up to and 

 including the 4-cell stage, and the nemertines; but, in this latter case, 

 blastomeres from the 4-cell stage are not perfectly totipotent, but give 

 rise to larvae lacking a few of the usual parts (Zeleny) . The group 

 of regulation eggs passes over without break into the group of mosaic 

 eggs. Thus in Amphioxus, totipotence exists only at the 2-cell stage, 

 as was shown by Wilson's experiments, and the same applies to 

 amphibia, but only under certain conditions, i.e. if each blastomere 



